Screening device in a rotatable drum intended for grinding and/or mixing material



2 Nov. 23, 1965 P. A. H. H SON FAHLSTRQM E SCREENING DEVICE IN A ROTA FOR GRINDING AND/OR M TAL 3,219,284 BLE DRUM INT ENDED NG MATERIAL Filed June 5, 1963 Per A. H- H-son Fahls-kr'om Erik I- Wilhelmssoh Rssar N Sve wsson LOMMM Hti-or'neys United States Patent 3,219,284 SCREENING DEVICE IN A ROTATABLE DRUM ENTENDED F011 GRINDING AND/OR MIXING MATERIAL Per A. H. H:son Fahlstriirn and Erik I. Wilhelmsson,

Boliden, and Assar N. Svensson, Ersmark, Sweden, assignors to Boiidens Gruvaktiebolag, Stockholm, Sweden, a joint-stock company limited, and Slrelleftea Gummifabriks AB, Ersmark, Sweden, a joint-stock company limited Filed June 5, 1963, Ser. No. 285,708 Claims priority, application Sweden, June 15, 1962, 6,711/62 4 Claims. (Cl. 241-70) The present invention relates to a screening device in a rotatable drum intended for continuously grinding and/ or mixing materials.

One or more screening operations are often included in grinding and mixing materials, simultaneously, or in connection with, the grinding or mixing operation. The use of rotating, horizontally mounted barrels, both for grinding and for mixing, has long been known. Known tumbling mills are ball and pebble mills, rod mills and autogenous mills. The first three of these operate with foreign grinding bodies, e.g. steel balls, pebbles, steel rods, and mills of the fourth kind operate with their own material as grinding bodies. These mills are usually called autogenous mills and grinding with them is called autogenous or rock grinding.

The barrel in which the grinding or mixing is carried out, consists of a normally cylindrical and/or conically tapering shell, horizontally or substantially horizontally mounted, and provided with openings through the heads for feeding material to be ground and discharging the ground product respectively. The barrel is usually mounted on trunnions for the feed and discharge respectively.

In all kinds of grinding, Whether with foreign grinding bodies or with grinding bodies of their own material, it is endeavored to reduce the grinding material as quickly as possible to the desired particle size. In grinding however, the grinding charge comes to consist of a variety of all particle sizes from the coarsest lumps of ore just received and still not having undergone any reduction in size, to the finest material forming the final ground product.

It is known that the tumbling grinding charge in a feedthrough mill of the rotating type has a principally homogenous particle composition throughout the entire space of the grinding drum, from the receiving end to the discharge end. In order to prevent the coarser, incompletely ground material, from being discharged with the material which has been completely ground, measures must be taken in the drum to separate the fine material from the coarse. To this end, it is known to arrange, in the vicinity of the discharge end, an intermediate screen wall dividing the chamber into two sections, only the fine material being able to pass through the screen apertures and be discharged, while the material in the section on the other side of the intermediate screen Wall in the grinding drum is subjected to further grinding until the particles have been reduced to a size which permits their passage through the screen openings into the section at the discharge end of the mill.

It is also possible to arrange additionally one or more intermediate screen walls of the kind described in spaced relation axially of the mill drum, the screen openings of each intermediate wall being successively diminished towards the discharge end of the mill. In this Way, with respect to the screen analysis, a more well-defined product can be discharged, meaning reduced work in subsequent classifiers and reduced quantities of oversized particles returned to the grinding circuit.

What has been stated above in connection with grinding and mixing materials belongs to the present state of technical practice and is well known to those skilled in the art. By and large, the matter stated applies to dry grinding as well as to wet grinding, and also to mixing operations including two or more grain or pulverulent products, possibly in the presence of liquid suspension media, or, mixing in combination with grinding, in which two or more solid materials, possibly with the addition of a liquid suspension medium, can be ground and mixed with each other in the mill and discharged as a finished mixed product, possibly in the form of a liquid suspension.

Even though the scope of the invention includes pure mixing operations, predominant interest lies in the grinding problem, it being observed, however, that when, in the following description, the principal interest is con cerned with grinding, the matter stated in relevant parts applies also to mixing operations.

In conventional mills of the type described, said intermediate screen wall and intermediate screen walls respectively consist of sector-shaped parts made throughout of wear-reistant steel which, by means of radial connecting members, are assembled to form a wall extending across the mill drum at right angles to the axis of rotation, which wall is fixed to the inner surface of the shell by bolts passing through the shell wall. A large number of apertures have been distributed over the surface of the sector-shaped parts, the apertures being round or having the form of more or less narrow slots which can lie either peripherally or radially. To prevent clogging, the apertures are usually widened in the direction of the discharge end of the mill. The radial connecting members, which likewise consist of Wear-resistant steel, protrude from the surface of the sector-shaped screen elements and serve therewith simultaneously as lifters for the charge tumbling in the grinding drum, whereby the wear as the surface of the screen elements is reduced because of the slipping of the grinding charge. For the same reason, similar lifters are arranged at the cylindrical inner surface of the mill drum and can therewith also serve as retainers for the lining of the mill drum.

Normal mills of the type described, which are provided with one or more intermediate screen walls made of steel and with steel lifters, have proved to be impaired by several disadvantages which reduce the effectiveness of the mill. The most fundamental inconvenience is the great wear and tear on both the screen elements and the lifter members, this necessitating relatively frequently recurring stoppages to change parts that are worn out. Another inconvenience is clogging of the apertures, the capacity of the mill being successively diminished until the mill must be shut down from operation in order to clean the screen plates. This applies particularly to ball mills where the apertures are clogged by chips and splinters from the balls.

According to the present invention it has now proved that the above disadvantages of normal rotation mills and mixing drums can be eliminated or reduced to a minimum if the sector-shaped screen elements consist throughout of an elastic material. As such, according to the invention, natural and synthetic rubber and all such synthetic elast-omers as are known for their good strength properties and particularly their resistance to wear, can be used. For mill drums intended for grinding ores and various types of stone, natural or synthetic rubber is preferably chosen, the properties of which are modified in a known manner by the admixture of reinforcing fillers (e.g. carbon black, highly dispersed silica), mineral oil, wood fiour, etc.

A rubber of the type used in vehicle tires can be considered to meet the demands which must be placed upon an elastic material which, according to the invention, is suggested for use as a material for an intermediate screen wall.

It has further proved that the advantages of the invention are not fully realised if only the screen element-s consist of rubber, as the radial connecting members or lifters are likewise subjected to the wearing efiect of the tumbling charge, and therefore these too should preferably consist of rubber. If, moreover, the grinding drum is lined with rubber in a known manner and is provided at its inner periphery with longitudinal, rubber-coated lifters, the complete mill drum will display, internally, a rubber surface with which the tumbling charge comes into contact.

The invention will be further explained below with reference to the attached drawing, in which FIGURE 1 shows an axial section through the discharge part of -a grinding drum intended for au-togenous grinding, FIG- URE 2 is a perspective view, partly in section, of the intermediate screen wall arranged at the discharge end of the mill drum, and FIGURE 3 is a section on a larger scale, seen in section IIIIII of FIGURE 1, showing the manner in which two sector-shaped screen elements are held together by a radial lifter.

In the drawing a mill barrel is shown, consisting of a cylindrical shell 1, a feed head (not shown) and a discharge head 2, which heads can be plane or slightly conical. The shell is provided with trunnions by means of which it is mounted for rotation around a horizontal axis of rotation. In the figure, trunnion 3 is shown on the discharge head. The cylindrical shell of the grinding barrel is suitably provided internally, in a known manner, with a rubber lining 4 and horizontal rubber lifters 5, i.e. operating parallel to the generating line of the cylindrical shell. In the discharge section of the barrel, an intermediate screen wall 6 is arranged, comprising a number of preferably sector-shaped screen elements 8 provided with a multiplicity of through apertures or slots 7, consisting throughout of rubber .or another suitable elastic material, which screen elements are retained by radial pairs of fastening ribs 9, 10, one on each side of the screen element, which mounting ribs are fixed by their outer ends to the cylindrical shell, suitably by means of bolts 13.

In the radial edges of the sector-shaped screen elements 8, which edges face the fastening ribs 9, 10, there are longitudinal recesses or grooves, whereby beads 11, 12 are formed and are retained in pairs by fastening ribs 9, 19 which for this purpose are bent longitudinally in conformity with the shape of the beads. The screen elements are joined by means of bolts 13 which, on being tightened, force the fastening ribs 9, towards each other, these in turn pressing tightly around the beads 11, 12 and retaining them in a firm grip. In FIGURE 1, an intermediate screen wall 6 is shown at the discharge end of the mill drum, and for this reason is constructed otherwise than if it were to be situated farther in the drum. The intermediate screen wall 6 at the discharge end of the mill is thus provided with a central, circular aperture 14 and from the edge of this opening there extend-s through the discharge trunnion 3 an outwardly tapering, conical part 15, likewise made of rubber. Between the conical discharge head 2 and the intermediate screen wall 6 a space is formed which is divided by radial Wall-s 18 and 19, in a known manner, into a number of sector-shaped scoop chambers into which the pulp strains when the chambers are below the pulp level in the mill during rotation of the grinding drum, and from which the pulp fl-ows out and leaves the mill when the chambers are raised during rotation. The details do not constitute objects of the invention and have therefore not been drawn. The intermediate screen Wall 6 is suitably fixed in such a way that the bolts 13 are allowed to pass out through the head 2, they being tightened with nuts on the outside, wherewith, for example, the above-mentioned radial walls 18 and 19 serve as distance members.

The fastening ribs 9, 10 mentioned above, not only hold the sector-shaped screen elements 8 together but also serve to support the lifting members 16 which, according to the invention, are made of rubber and are preferably securely vulcanized to the mounting ribs. As is clear from FIGURE 3, the heads of the bolts 13 are sunk in apertures 17 in the rubber lifting members 16, and in order to prevent wear at the edge of the hole, it can suitably be plugged and vulcanized with a rubber plug (not shown). As is further evident from FIGURE 3, the rubber lifting members 16 have a rectangular cross-section with a rounded corner. After the mill has been operating for some time, the edge facing the flow of material when the mill barrel rotates, starts to be worn away at a greater rate than the edge on the sheltered side of the grinding material flow. As wear then takes place at a slower rate, in order to save rubber, the rubber rib can from the beginning be given a corresponding shape.

The holes or channels 7 in the screen elements Preferably have a rectangular cross-section, but can also be of round or oval cross-section, the short side of the rectangular channel, the diameter of the circular channel or the short axis of the oval cross-section, not being greater than about 20% of the thickness of the screen element. As is shown in FIGURE 1, the channels also widen in the direction of the discharge end of the mill, the aperture angle or angle of taper being at the most 1:20 or suitably about 1:50.

Only one intermediate screen wall has been shown in the drawing at the discharge end of the mill barrel. In ball mills of the multi-chamber type with peripheral discharge, however, several intermediate screen walls can be arranged in the mill barrel in order to divide it into several chambers. These intermediate screen walls then have no central aperture as shown in the figure, but ex tend across the entire cross-section of the barrel. The screen holes in the respective intermediate wa-lls then suitably diminish in size from wall to wall, whereby during the grinding process a classification of the grinding material in the various chambers takes place, from a coarser to a finer ground product.

The invention ofiers many substantial advantages as op-' posed to conventional mill and mixing drums. The rub- .ber, especially in wet grinding, has proved to be an extraordinarily wear-resistant material which can successfully compete with steel in the question of length of working life and which, moreover, has the great advantage of being light in comparison with steel. From this it follows that the work of mounting is substantially facilitated, as is the changing of parts during repairs. Owing to the reduced weight-especially if, in addition, the shell is lined with rubber instead of having a steel linerit is not necessary to make such great demands on the fundament. Owing to the reduced weight moreover, the barrel requires considerably less electric power, andsmaller drivemotors can be used. Despite these substantial advantages, however, one of the most prominent is that the barrel, owing to the elastic material in the intermediate screen wall or walls and the widening of the screen holes, can 'be driven longer without shut-downs caused by clogging. Owing to the fact that the walls of the holes are yielding, splinters from grinding balls or oversized particles do not become lodged in the holes, but are forced through the holes by following particles, while the holes in screen elements made of steel soon become clogged to such a degree that the mill must be stopped at relatively short intervals to clean the holes. A further advantage is obtained from the materials being yielding, and that is that the slots or holes in the screen elements can be made narrower or smaller, whereby a finer classification of the finished and incompletely ground material can be attained. As the pulp discharged from a wet grinding mill always contains oversized particles and therefore must be subjected to a classification to return the oversized particles to the mill, it is obvious that, owing to the fine classification achieved according to the invention, it is not necessary to use large and expensive classification devices, it being possible instead to discharge a pulp right from the mill, which can be pumped and which, without special coarse material classification, can be pumped directly to the hydrocyclone separators, which are preferably used nowadays.

Having now described the invention, what we claim as new and desire to secure by Letters Patent is:

1. A screening device in a rotatable drum intended for grinding and/ or mixing material, the drum consisting of a shell With a receiving head and a discharge head and being internally provided with an intermediate screen wall for separating the material therein into a coarse and a fine material fraction, said intermediate screen wall consisting of a number of sector-shaped screen elements of rubber securely retained by radially extending fastening ribs, the radial edges of the sector-shaped screen elements facing the fastening ribs being provided with beads, the fastening ribs being bent longitudinally in conformity with the shape of the beads and being securely engaged therewith.

2. A screening device as claimed in claim 1 in which said fastening ribs also serve as a support for a lifter member of rubber securely vulcanized to the fastening rib, the lifter member extending outwardly from the screen element.

3. A screening device as claimed in claim 1 in which the screen elements are retained on both sides by separate, interchangeable fastening ribs.

4. A screening device as claimed in claim 3 mounted at the discharge end of the drum, the fastening bolts passing through, in turn, a first fastening rib, the beads of two adjacent screen elements and a second fastening rib opposite to said first fastening rib, the radial walls dis posed at the inside of the inside of the discharge head forming sector-shaped scoop chambers, and holes in the discharge head.

References Cited by the Examiner UNITED STATES PATENTS 1,381,987 6/1921 Gordon et al. 241- X 1,690,493 11/1928 Marcy 24l183 X 2,058,257 10/1936 Porteous 241-102 2,611,546 9/1952 Posselt 241183 2,931,583 4/ 1960 Johnson 241-70 3,107,867 10/1963 Svenson 241183 FOREIGN PATENTS 108,053 7/1937 Australia.

ROBERT c. RIORDON, Primary Examiner.

J. SPENCER OVERHOLSER, Examiner. 

1. A SCREENING DEVICE IN A ROTATABLE DRUM INTENDED FOR GRINDING AND/OR MIXING MATERIAL, THE DRUM CONSISTING OF A SHELL WITH A RECEIVING HEAD AND A DISCHARGE HEAD AND BEING INTERNALLY PROVIDED WITH AN INTERMEDIATE SCREEN WALL FOR SEPARATING THE MATERIAL THEREIN INTO A COARSE AND A FINE MATERIAL FRACTION, SAID INTERMEDIATE SCREEN WALL CONSISTING OF A NUMBER OF SECTOR-SHAPED SCREEN ELEMENTS OF RUBBER SECURELY RETAINED BY RADIALLY EXTENDING FASTENING RIBS, THE RADIAL EDGES OF THE SECTOR-SHAPED SCREEN ELEMENTS FACING THE FASTENING RIBS BEING PROVIDED WITH BEADS, THE FASTENING RIBS BEING BENT LONGITUDINALLY IN CONFORMITY WITH THE SHAPE OF THE BEADS AND BEING SECURELY ENGAGED THEREWITH. 